Force -transmision unit comprising speed -dependent hydraulic clutch and centrifugal force compensation

ABSTRACT

A power transmission unit with a hydraulic coupling dependent on a rotational-speed difference, in which, when a rotational-speed difference occurs between the input member ( 1 ) and the output member ( 6 ), a hydrostatic displacement machine ( 20 ) produces in a pressure space ( 34 ) a pressure that acts on a piston ( 27 ) acting on a friction clutch ( 31 ), has a housing ( 4 ). To compensate for the centrifugal force acting on the working fluid in the pressure space ( 34 ), at least one centrifugal-force element ( 11, 12, 13 ) is provided in the housing, exerting on the piston ( 27 ) a force counter to the pressure produced by the centrifugal force in the pressure chamber ( 34 ).

The invention relates to a power transmission unit with an input memberand an output member and a hydraulic coupling dependent on arotational-speed difference, in which, when a rotational-speeddifference occurs between the input member and the output member, ahydrostatic displacement machine produces in a pressure space a pressurethat acts on a piston acting on a friction clutch, the friction clutchhaving first and second disks connected in terms of drive to the inputmember and the output member respectively, and one of the membersforming a housing that contains the displacement machine.

Power transmission units of this kind are used especially in drivetrains of motor vehicles, preferably all-wheel-drive vehicles; eithertogether with a differential, the hydraulic coupling limiting thedifferential action, or to drive the second driven axle, the torquetransmitted depending on the difference between the wheel speed and thedrive shaft connected to the wheels of the other axle. The pressureproduced by the displacement machine acts on a clutch, preferably amulti-plate clutch. This action can be influenced by means of variousvalves, whether these are automatically acting valves or valves actuatedby an external control system.

U.S. Pat. No. 5,536,215 has disclosed a power transmission unit of thiskind, as has Austrian Utility Model 2964. In these and all such powertransmission units, the pressure space in which the pressure acting onthe piston is built up is in the rotating housing. As a result, theoperating fluid contained in this housing is subject to a centrifugalforce, which increases and thus distorts the pressure prevailing in thepressure chamber as a function of the rotational speed. This isparticularly disruptive if the pressure is dependent on arotational-speed difference and is supplied by a hydrostaticdisplacement machine, and this applies in both possible cases: if, inthe first case—that of an unregulated coupling—there are no controlvalves, compensation is impossible; and if, in the second case, controlvalves intended to depressurize the pressure space for disengagement areprovided, this is not possible at higher absolute rotational speedsbecause the discharge line adjoining the control valve has to end in asmaller radius. However, the pressure there is always less than in thepressure chamber, owing to centrifugal force.

It is therefore the object of the invention to eliminate thesedisadvantages corresponding to the special features of couplings of thegeneric type. The influence of centrifugal force should be at leastpartially compensated for to a necessary extent.

According to the invention, at least one centrifugal-force element isprovided for this purpose in the housing, exerting on the piston a forcethat is the square of the rotational speed and acts counter to thepressure acting on the piston. By virtue of the fact that it is likewisesituated in the housing, compensation to a specifiable extent ispossible in all rotational-speed ranges without any outlay on regulationsystems, given appropriate design. It is thereby possible to establish aspeed dependence of the transmitted torque corresponding to therequirements as regards driving dynamics. The extent of compensationranges from partial compensation and full compensation toovercompensation. In this arrangement, the transmitted torque falls asthe speed increases, giving better traction at low speed and improvedinteraction with electronic brake systems (e.g. ABS) at high speed.

In an advantageous design, the at least one centrifugal-force element isa flyweight (claim 1). Compensation of centrifugal force is thusperformed in a purely mechanical way, and, in a preferred embodiment,the centrifugal-force element is part of a two-armed lever, one leg ofwhich is the flyweight and the other lever of which is a pressure finger(claim 3). The levers, of which there are three for example, are verysimple and can be accommodated in the housing with only slight designchanges. This is the simplest solution and can even be retrofitted toexisting couplings.

The other design comprises the centrifugal-force element being anannular space that contains an operating fluid and rotates with thehousing (claim 4). This is a hydraulic method of compensating forcentrifugal force. Since there is sufficient operating fluid in andaround the coupling, there is no problem with supplying it.

In a first advantageous embodiment of this other design, the rotatingannular space is formed by a cylindrical sleeve surrounding the housingand having a wall in the form of a circular ring normal to the axis andby a wall, normal to the axis, of the housing, and the sleeve isconnected to the piston and can be displaced in an axial direction(claim 5).

In this way, the annular space is bounded on one side by a displaceablewall and on the other side by a nondisplaceable wall of the housing. Theliquid level in the annular space is determined by the inner radius ofthe wall in the form of a circular ring normal to the axis. Thecentrifugal force acting on the working medium in the annular spacepushes apart the walls normal to the axis. This compensating force istransmitted to the piston by the displaceable sleeve.

In a second advantageous embodiment of this other design, the radiallyoutermost zone of the rotating annular space is connected via a passageto a compensation pressure space on the opposite side of the piston fromthe pressure space (claim 6). The annular space and the passage can alsobe provided within the housing. It is even possible, by means of valvesassociated with the passage, to achieve special effects in terms ofdriving dynamics.

A particularly elegant solution is for the compensation pressure spaceto be formed by an annular cylinder in the housing and by an annularcontinuation on the opposite side of the piston from the pressure space(claim 7).

The invention is described and explained below with reference tofigures, in which:

FIG. 1 shows a longitudinal section through a device according to theinvention in a first embodiment,

FIG. 2 shows a longitudinal section through a device according to theinvention in a second embodiment, and

FIG. 3 shows a longitudinal section through a device according to theinvention in a third embodiment.

In FIG. 1, the input member is denoted by 1 but it could also be theoutput member, to which a shaft 2, indicated in broken lines, is flangedby means of bolts, which are merely indicated. It comprises a frontplate 3, an essentially cylindrical housing 4, which is connectedintegrally or in a fixed manner to the front plate 3, and an end plate5, which is connected releasably to the housing 4 for the purpose ofassembly, though in a leaktight way. The output member 6 (it could alsobe the input member) is a hollow shaft, into which a shaft that ismerely indicated is introduced by means of splines; it is supported inbearings 7 in the front plate 3 and the end plate 5 of the input member1 and can be sealed off relative to the latter by means of seals 8.Simple sealing rings are sufficient because the rotational-speeddifference is very small on average. 9 denotes the axis of rotation orcenter line.

Within the housing 4, there is a hydrostatic displacement machine 20,which comprises an inner part 21 and an outer part 22. The first ofthese is connected in a rotationally fixed manner to the output member6, while the second is connected to the input member 1 and, morespecifically, to the housing 4. The corresponding coupling teeth aremerely indicated. Extending between the inner part 21 and the outer part22 is a working space 23, which is supplied via an intake passage 24 ina manner that is not shown. Adjoining the hydrostatic displacementmachine 20 on the other side is an insert 25, which contains a pressurepassage 26 and a piston 27, which is acted upon by the pressurized fluidsupplied via the pressure passage 26 and, with the insert 25, delimits apressure space 34. Some of this pressurized fluid can be directed intothe space, which contains a clutch 31, via a throttle valve 28 by apiston 27, a number of inner plates 29 and outer plates 30 beingarranged in said space. The first of these are connected to the outputmember 6 in a way that prevents relative rotation but allowstranslation, while the second are connected in the same way to thehousing 4 of the input member 1.

For the purpose of mounting a device for compensating the force exertedon the piston 27 by the centrifugal force in the pressure chamber 34,the housing 4 here has a plurality of apertures 10, which aredistributed around the circumference and through which two-armed angledlevers 12 reach. One leg of such a lever is constructed as a flyweight11, while the other is constructed as a pressure finger 13, whichengages in a recess 14 on the opposite side of the piston 27 from thepressure space 34. Instead of a pivot passing through the two-armedlever 12, a bearing edge 15, on which a bearing shoulder 16 on the rearside of the pressure finger 13 is supported, is provided here on theaperture 10 in the housing 4. This ensures that the lever 12 does notfly off. A projection 18, which is held by an end stop 17 when theoutermost permitted position of the flyweight 11 is reached, can beprovided on the outermost end of the flyweight 11.

FIG. 2 shows a different design. Here too, the housing 4 has a pluralityof apertures 10 distributed around the circumference, through whichradial pins 40 inserted into the piston 27 extend outward and areconnected to a cylindrical sleeve 41 surrounding the housing 4 all theway round. They can transmit a force in the axial direction between thesleeve 41 and the piston 27. The cylindrical sleeve 41 extends towardthe left in the figure, projects beyond the housing 4 and ends in a wall42 in the form of a circular ring normal to the axis. An annular space44 is thus formed between this wall and a wall 43, normal to the axis,of the housing 4. This annular space is sealed off by means of a sealingring 45 between the housing 4 and the sleeve 41 and contains workingfluid to a level determined by the inside diameter of the wall 42.

When the housing 4 is rotated, this liquid surface 46 becomes acylindrical surface. During rotation, the centrifugal force in thisannular space 44 gives rise to a pressure that pulls the wall 43 of thesleeve 41 to the left in the exemplary embodiment illustrated and thus,in turn, exerts on the piston 27, via the pins 40, a force thatcompensates for the centrifugal force in the pressure space 34. Thedesign and position of the annular space 44 can also be modified. Theessential point is that an axial force counter to the force acting onthe piston 27 in the pressure space 34 arises.

According to the variant in FIG. 3, the connection between the annularspace and the piston can also be established hydraulically. For thispurpose, an annular space 50 is again provided, on the opposite side ofthe piston 27 from the pressure space 34 and within the housing 4 in theexemplary embodiment shown. The annular space 50 is kept filled from theinterior of the clutch space via a feed hole 51, a drain hole 52ensuring that a constant (cylindrical) liquid surface 53 is maintained.The pressure produced by the centrifugal force in the annular space 50acts via an axial passage 54 (or a plurality of such passages) on anannular cylinder 55. This is likewise formed in the housing 4 andaccepts an annular continuation 56 of the piston 27 in a sealing manner.With the annular cylinder 55, it forms a compensation pressure space 57.There, the pressure acts on the surface 58 in the form of a circularring and thus compensates for the action of the pressure prevailing inthe pressure chamber 34.

It is possible to modify many details of the exemplary embodimentsillustrated while remaining within the scope of the invention. Thus theconstruction of the hydrostatic displacement machine can vary verywidely, both as regards the shape of its rotors and as regards theirarrangement in the housing 4. Finally, the power transmission unit canbe arranged at various points within the drive train, in particularahead of or after the axle differential in the power flow. It can alsobe arranged within a housing containing the axle differential.

1. A power transmission unit with an input member and an output memberand a hydraulic coupling dependent on a rotational-speed difference, inwhich, when a rotational-speed difference occurs between the inputmember (1; 6) and the output member (6; 1), a hydrostatic displacementmachine (20) produces in a pressure space (34) a pressure that acts on apiston (27) acting on a friction clutch (31), the friction clutch havingfirst and second disks connected in terms of drive to the input memberand the output member respectively, and one of the members (1; 6)forming a housing (4) that contains the displacement machine, wherein atleast one centrifugal-force element (11, 12, 13; 41, 44; 50, 55) isprovided in the housing, exerting on the piston (27) a force counter tothe pressure produced by the centrifugal force in the pressure chamber(34).
 2. The power transmission unit as claimed in claim 1, wherein theat least one centrifugal-force element is a flyweight (11).
 3. The powertransmission unit as claimed in claim 2, wherein the centrifugal-forceelement is a two-armed lever (12), one leg of which forms the flyweight(11) and the other lever of which forms a pressure finger (13).
 4. Thepower transmission unit as claimed in claim 1, wherein thecentrifugal-force element is an annular space (44; 50) that contains anoperating fluid and rotates with the housing (4).
 5. The powertransmission unit as claimed in claim 4, wherein the rotating annularspace (44) is formed by a cylindrical sleeve (41) surrounding thehousing (4) and having a wall (42) in the form of a circular ring normalto the axis and by a wall (43), normal to the axis, of the housing (4),and wherein the sleeve (41) is connected to the piston (27) and can bedisplaced in an axial direction.
 6. The power transmission unit asclaimed in claim 4, wherein the radially outermost zone of the rotatingannular space (50) is connected via a passage (54) to a compensationpressure space (57) on the opposite side of the piston (27) from thepressure space (34).
 7. The power transmission unit as claimed in claim6, wherein the compensation pressure space (57) is formed by an annularcylinder (55) in the housing (4) and by an annular continuation (56) onthe opposite side of the piston (27) from the pressure space (34).